Insulation capacity of three bark types of temperate Eucalyptus species

• Bark thickness is the most important bark trait for heat resistance of a tree.
• Conductance of heat differs among bark-types of Eucalyptus species.
• Density of bark does not have a critical impact on heat resistance.
• A novel experimental design identifies the role of bark moisture as ‘coolant’ and as ‘conductor’.

Fire plays an increasingly important role in management of native forests and plantations around the world. Thick tree bark represents the most important defence against surface fires, although other bark traits (bark-type, moisture content, density) are also involved. The interplay of bark traits to reduce heat-induced cambium necrosis and related tree death remains poorly understood. Here we introduce a novel method using multiple sensors to accurately measure conductance of heat through the periderm and secondary phloem and to detect how individual bark traits influence the transfer of heat through those tissue types. We employed this method to document the capacity for bark insulation of three Eucalyptus species with different bark-type from temperate Australia, simulating a ‘worst-case’ scenario (750 °C bark surface temperature for 900 s). Thickness of these different bark types ranged from 3 to 65 mm. Our results clearly show the importance of thickness and type of bark for prevention of cambium necrosis due to heat. Coefficients of determination that describe how bark thickness correlates with time to reach lethal temperatures (>60 °C) at the cambium ranged from 0.61 (least effective: E. tricarpa, ironbark-type bark, average moisture content = 34%, average bark density = 0.58 g cm−3) to 0.94 (most effective: E. leucoxylon, gum-type bark, average moisture content = 54%, average bark density = 0.42 g cm−3) and both followed linear and curvilinear trajectories. The cooling effect of water in the periderm was found to slow conduction of heat towards the cambium. This effect has not previously been documented by empirical measurements and may have significant implications to survival of trees during “cooler” prescribed fires. Our study highlights between-species variation in ability to withstand heat from surface fires. Fire temperatures and duration thus have considerable capacity to change species composition of these Box-Ironbark forests via mortality.